Element partitioning between carbonates and high-pressure phases in mafic and ultramafic C-O-H bearing systems

Element partitioning between carbonates and coexisting high pressure phases plays a critical role in unravelling petrogenetic processes of eclogitic rocks, peridotites, diamond-bearing rock suites, and carbonatitic magmas. Experiments were carried out on MORB and lherzolitic bulk compositions, from gel starting materials, using piston-cylinder and multianvil machines at P from 1 to 6 GPa and T from 650 C to 1050 C, in the presence of C-O-H fluids generated from starting mixtures of oxalic acid dihydrate and silver oxalate. fH2 was buffered at NNO, OH (X, COH) and hematite-magnetite HM, OH (X, COH) buffers using a double capsule technique (notation after [1]). Although element partitioning is examined in synthesis experiments and reversals are hindered by low-reactivity of starting materials composed of crystalline mixtures, excellent T-P correlation of Mg/Fe partitioning between garnet and pyroxenes reveals approach to equilibrium conditions in both mafic and ultramafic systems. New results were integrated with a dataset from other high-pressure synthesis experiments recently published on carbonate-bearing mafic and ultramafic systems. A significant temperature dependence of Ca-Mg exchange reactions and the negative correlation of lnKFe􀀀Mg vs. 1/T between Ca-carbonate and garnet allow formulating thermometric expressions suitable for the estimation of equilibrium crystallization temperatures in a variety of bulk compositions. Differences between mafic and ultramafic systems will be highlighted. Comparison with element partitioning predicted by thermodynamic calculations will also be shown. [1] Eugster & Skippen, 1967, Res. Geochem. 2, 492–526.